forge install-plugin gives NullPointerException

Typing the command:
forge install-plugin arquillian
hangs a long time, and then gives the below NullPointerException. Could we at least get a more detailed error message?

java.lang.NullPointerException
at org.jboss.forge.shell.plugins.PluginManager.installFromProject(PluginManager.java:160)
at org.jboss.forge.shell.plugins.builtin.ForgePlugin.installFromGit(ForgePlugin.java:387)
at org.jboss.forge.shell.plugins.builtin.ForgePlugin.installFromIndex(ForgePlugin.java:206)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.jboss.forge.shell.command.Execution.perform(Execution.java:160)
at org.jboss.forge.shell.command.fshparser.FSHRuntime.run(FSHRuntime.java:109)
at org.jboss.forge.shell.command.fshparser.FSHRuntime.run(FSHRuntime.java:47)
at org.jboss.forge.shell.ShellImpl$ExecutorThread.run(ShellImpl.java:796)
at org.jboss.forge.shell.ShellImpl.execute(ShellImpl.java:819)
at org.jboss.forge.shell.ShellImpl.doShell(ShellImpl.java:609)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.jboss.weld.bean.proxy.AbstractBeanInstance.invoke(AbstractBeanInstance.java:48)
at org.jboss.weld.bean.proxy.ProxyMethodHandler.invoke(ProxyMethodHandler.java:125)
at org.jboss.forge.shell.ShellImpl$Proxy$$$_WeldClientProxy.doShell(ShellImpl$Proxy$$$_WeldClientProxy.java)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.jboss.weld.util.reflection.SecureReflections$13.work(SecureReflections.java:305)
at org.jboss.weld.util.reflection.SecureReflectionAccess.run(SecureReflectionAccess.java:54)
at org.jboss.weld.util.reflection.SecureReflectionAccess.runAsInvocation(SecureReflectionAccess.java:163)
at org.jboss.weld.util.reflection.SecureReflections.invoke(SecureReflections.java:299)
at org.jboss.weld.introspector.jlr.WeldMethodImpl.invokeOnInstance(WeldMethodImpl.java:188)
at org.jboss.weld.introspector.ForwardingWeldMethod.invokeOnInstance(ForwardingWeldMethod.java:59)
at org.jboss.weld.injection.MethodInjectionPoint.invokeOnInstanceWithSpecialValue(MethodInjectionPoint.java:198)
at org.jboss.weld.event.ObserverMethodImpl.sendEvent(ObserverMethodImpl.java:282)
at org.jboss.weld.event.ObserverMethodImpl.sendEvent(ObserverMethodImpl.java:265)
at org.jboss.weld.event.ObserverMethodImpl.notify(ObserverMethodImpl.java:234)
at org.jboss.weld.manager.BeanManagerImpl.notifyObservers(BeanManagerImpl.java:635)
at org.jboss.weld.manager.BeanManagerImpl.fireEvent(BeanManagerImpl.java:622)
at org.jboss.weld.manager.BeanManagerImpl.fireEvent(BeanManagerImpl.java:616)
at org.jboss.forge.shell.Bootstrap$1.run(Bootstrap.java:186)
at java.lang.Thread.run(Thread.java:722)